NEAR Mission Profile

Launch Phase - 17 February 1996
NEAR was launched on 17 February 1996, at 20:43 UT
(3:43 p.m. EST) from Pad 17-B at Cape Canaveral Air Station,
FL. using a three-stage Delta II-7925 expendable launch
vehicle.
The Delta II parking orbit had an altitude of 100 miles
(183 kilometers) and an inclination of 28.74 degrees. The
launch azimuth was fixed at 95 degrees. The coast period in
the parking orbit was relatively short (13 minutes), allowing
solar power to be used starting one hour after launch. The
injection burn, accomplished mainly by a four-minute burn of
the third stage solid motor, was entirely inside Earth's shadow.
Approximately 22 minutes after launch, the spacecraft
separated from the third stage. A yo-yo despin mechanism
simultaneously released the solar panels from their stowed
launch position and despun the spacecraft from a maximum of
69 rpm to a nominal rate of 0 rpm. Once the solar panels were
released, spring-loaded hinges deployed them to the on-orbit
configuration. The spacecraft exited the Earth's shadow 37
minutes after launch. From launch until this time, the
spacecraft was battery-powered. Because weight constraints
limited the size of the battery, only those components
considered mission critical during this phase were powered.
At third stage separation, responsibility for attitude control
passed from the Delta to the spacecraft's guidance and control
subsystem.
Cruise Phase
NEAR followed a so-called "Delta VEGA" trajectory to
provide the extra energy needed to accomplish the rendezvous
with Eros, which orbits the Sun at an angle of 10.8 degrees
to the ecliptic. "Delta V" stands for change in velocity;
"EGA" is an abbreviation for Earth Gravity Assist.
During the first few weeks of cruise, a series of
component functional tests checked the health of the
spacecraft. Also during this time, low-level burns were
performed to calibrate the propulsion system and to correct
for any trajectory errors.
After this checkout period the spacecraft maintained
minimal activity. Due both to limited electrical power
beyond 2 AU (186 million miles/300 million kilometers) and a
desire not to thermally stress the solar panels during
cruise, spacecraft operations Sunward of 1.5 AU (140 million
miles/225 million kilometers) and outward of 2 AU were
intentionally limited. All of the instruments were off. The
telemetry subsystem periodically sampled low-level
housekeeping and navigation data and stores the information
on the solid state recorder. Heaters were used to maintain
the temperature of the inactive systems.
The spacecraft maintained this hibernation mode except
during ground station contacts. Conducted during four-hour
passes, three times per week, the ground contacts permit the
mission operations team to analyze current spacecraft
health, upload the next week's series of command sequences,
and dump recorded telemetry.
NEAR maintained this routine until preparations
began for two critical mission events -- the Mathilde flyby
and the Deep-Space Maneuver -- in the June/July 1997
timeframe. In support of the increased activity, the DSN
provided continuous coverage from its 34-meter network
from June 20 to July 10, 1997. NEAR required 21 eight-
hour passes per week from these antennas instead of the
normal cruise coverage. A third major event during cruise --
the Earth swingby -- occurred in January 1998.
As part of preparations for the flybys, the
multispectral imager periodically pointed at the target and
an image was transmitted to Earth. This optical navigation
(OpNav) data was combined with ground tracking information to
calculate a flyby trajectory. The trajectory calculation was
refined throughout the flyby approach as more ground data
was taken and more images returned. Prior to the flyby, a
time-tagged command sequence was uploaded to the spacecraft
defining a time-ordered sequence of command to be executed
during the flyby. These commands include an open-loop
pointing trajectory and instrument data capture sequences.
Mathilde Flyby - 27 June 1997
NEAR's flyby of the 50 x 53 x 57 kilometer C-type
main belt asteroid 253 Mathilde took place on June 27, 1997,
at a distance from Earth of 2.2 AU (205 million miles/330
million kilometers). Closest approach distance was 1,200
kilometers and occurred at 12:56 UT. Although the approach
phase angle was almost 140 degrees, NEAR's imaging system
was able to obtain useful optical navigation images
beginning about three days before the encounter. OpNav
sequences were scheduled at four-hour intervals with each
sequence consisting of four pictures. Flyby speed was 9.93
kilometers/sec.
The primary science instrument was the camera, but
measurements of magnetic fields and mass were also made.
The whole illuminated portion of the asteroid was imaged
in color at about 0.6-mile (1-kilometer) resolution, with
the best monochrome views at some 660 to 980 feet (200 to
300 meters) resolution. As the spacecraft recedes from
Mathilde, a thorough search for satellites was
conducted but none were observed.
Deep Space Maneuver - 3 July 1997
The Deep Space Maneuver (DSM) was executed about
one week after the Mathilde flyby, on July 3, 1997. The DSM
represents the first of two major burns during the NEAR
mission of the 100-pound (450-Newton) bi-propellant
(hydrazine / nitrogen tetroxide) thruster. The spacecraft
was slowed by 625 miles/hour (279 meters/second). This
maneuver is necessary to lower the perihelion distance of
NEAR's trajectory, from 0.99 AU to 0.95 AU (92 million
miles/148 million kilometers to 88 million miles/142
million kilometers).
The DSM was conducted in two segments to minimize
the possibility of an overburn situation. The first segment,
DSM-1, provided 90 percent of the required delta-V of 279
m/sec and was performed with the high gain antenna in
operation to monitor critical engineering data.
Accelerometer measurements of DSM-1 were then be used to
update DSM-2, which will supply the remaining 10 percent of
the delta-V.
Earth Swingby - 22 January 1998
The next critical phase of NEAR's flight profile
occurred on January 22, 1998, when the spacecraft passed
by the Earth at an altitude of 540 kilometers at 07:23 UT.
This maneuver altered NEAR's heliocentric trajectory,
changing the inclination from 0.5 to 10.2 degrees and
reducing the aphelion distance from 2.17 to 1.77 AU (200
million miles to 165 million miles/325 million kilometers
to 265 million kilometers). Consequently, NEAR's
post-swingby trajectory virtually matched the inclination
and aphelion distance of Eros' orbit, which significantly
reduces the magnitude of the rendezvous maneuver.
An interesting aspect of the Earth flyby is that the
post-swingby trajectory remained over the Earth's south polar
region for a considerable time. This provided an
opportunity for NEAR to obtain some unique images of the
Antarctic continent. Also, because of its extreme southerly
declination, the spacecraft could be viewed continuously from
the DSN Canberra station for 71 days following the Earth
flyby. The first visibility from the Goldstone and Madrid
stations could not occur until 110 and 120 days,
respectively, after the flyby.
Asteroid 433 Eros Approach - 9 January 1999
First detection of Eros by the multispectral imager
occurred on 5 November 1998, approximately 200 days prior to
closest approach. Following this early observation, clusters
of images were obtained weekly for optical navigation and
for initial shape and rotation determination.
Beginning on Jan. 9, 1999, a series of four rendezvous
maneuvers with the main thruster were scheduled -- spaced
seven days apart -- to slow NEAR by 2,123 miles/hour
(949 meters/sec) to achieve a relative velocity between the
spacecraft and Eros of 11 miles/hour (5 meters/sec).
However, the first of the scheduled rendezvous burns on
20 December 1998 at 22:00 UT aborted due to a software problem.
Contact was lost immediately after this and was not
re-established for over 24 hours. The original mission plan
called for the burns to be followed by an orbit insertion burn
on 10 January 1999, but the abort of the first burn and loss of
communication made this impossible. A new plan was put into
effect in which NEAR flew by Eros on 23 December 1998 at
18:41:23 UT at a speed of 0.965 km/s and a distance of 3827 km
from the center of mass of Eros. Images of Eros were taken by
the camera, data was collected by the near IR spectrograph, and
radio tracking was performed during the flyby. Orbit insertion
around Eros is now planned for 14 February 2000 after a 13
month heliocentric orbit which closely matches the orbit of
Eros.
On 3 January 1999 a large bipropellant thruster burn was
executed to close the gap between NEAR's orbital speed and that
of Eros. On 20 January a hydrazine thruster burn was completed
to fine-tune the spacecraft's trajectory and speed. The last
major trajectory correction was completed on 12 August with a
2-minute burn of the hydrazine engine, slowing the spacecraft to
188 mph relative to Eros
Asteroid 433 Eros Orbit Insertion occurred on 14 February 2000
at 15:33 UT (10:33 EST). The thruster burn put NEAR into a
327 x 450 km near-polar orbit with a period of 27.6 days.
The schedule now calls for the following:
Feb. 14 to April 30, 2000
High-Orbit Phase. NEAR orbits Eros at distances decreasing
from 311 to 31 miles (500 to 50 kilometers) from the center
of the asteroid.
March 10, 2000
NEAR spacecraft descends to 125-mile (200-kilometer) orbit.
April 10, 2000
NEAR reaches orbit of 62 miles (100 kilometers).
April 30, 2000
NEAR arrives at a polar orbit of 31 miles (50 kilometers),
where the spacecraft spends 100 days.
April 30 Ð Aug. 27, 2000
Low-Orbit Phase. NEAR travels in nearly circular orbits at
about 31 miles (50 kilometers) from Eros. The
X-Ray/Gamma-Ray Spectrometer measures element abundances,
which will help to determine the relationship between
meteorites and asteroids.
July 6, 2000
For the first time since NEAR arrived at Eros, all of Eros,
excluding deep polar craters, is illuminated by the sun.
Sunlight shines directly over Eros' equator as the sub-solar
point moves south.
Aug. 27 Ð Dec. 20, 200
High-Orbit Phase. NEAR travels in orbits of 31 to 311 miles
(50 to 500 kilometers) from Eros. During this period, the
retrograde orbit shifts from nearly polar to nearly
equatorial, where NEAR travels opposite the direction of
Eros' spin.
Oct. 15, 2000
Zero-phase measurements occur using the Near-Infrared
Spectrometer as the spacecraft flies between Eros'
southern hemisphere and the sun.
Dec. 20, 2000
Low-altitude operations begin as the spacecraft passes
within 21 miles (35 kilometers) or closer during each
orbit.
Feb. 14, 2001
Mission ends.
The irregular shape of Eros requires that NEAR remain
in retrograde orbit relative to the asteroid spin. Prograde
orbits tend to be unstable in the sense that the spacecraft
would typically be ejected from orbit or caused to impact
the surface. An orbital plane flip maneuver at approximately
mid-mission is required to maintain a retrograde orbit.
When data are to be downlinked, the spacecraft will be
slewed if necessary to point the high-gain antenna at Earth.
The instruments face 90 deg. from the direction of the antenna,
so they can point at Eros as the spacecraft rolls in its
orbit. All or any combination of the instruments can operate
simultaneously, taking data and storing data on the solid
state recorders. The spacecraft also can take data and
downlink data simultaneously, although the instruments can
be pointed at the asteroid for only a small portion of the
downlink periods.
Navigation constraints at Eros are designed to permit
the spacecraft to orbit as low as possible for as long as
possible to accomplish scientific objectives. They include:
- Spacecraft orbit should be safe and stable for a
timespan of weeks.
- Normally there should be no less than seven days
between propulsive maneuvers.
- Total rendezvous delta-V expenditure should be less than
224 miles/hour (100 meters/sec).
- Sun pointing angle must be limited to less than 30 degrees
because of power and payload pointing constraints resulting
from the fixed mounting of the solar arrays and instruments.
This information has been adapted from the NEAR press kit and
the mission timeline.